Conjugated linoleic acid (CLA) has become the focus of numerous research programs which seek to capitalize on its nutritional, therapeutic, and pharmacologic properties.
The rearrangement of the double bonds of linoleic acid to conjugated positions has been shown to occur during treatment with catalysts such as nickel or alkali at high temperatures, and during auto oxidation. Theoretically, eight possible geometric isomers of 9,11 and 10,12 octadecadienoic acid (c9,c11; c9,t11; t9,c11; t9,t11; c10,c12; c10,t12; t10,c12 and t10,t12) would form from the isomerization of c9,c12-octadecadienoic acid. A general mechanism for the isomerization of linoleic acid was described by J. C. Cowan in JAOCS 72:492–99 (1950). It is believed that the double bond is polarized by the result of a collision with an activating catalyst. The polarized carbon atom and its adjoining carbon are then free to rotate and the forces are such as to make the deficient carbon atom essentially planar. When the system then moves to relieve these forces set up as a result of the collision, both cis and trans isomers are formed. The formation of certain isomers of CLA is thermodynamically favored. This is due to the co-planar characteristics of the five carbon atoms around the conjugated double bond and a spatial conflict of the resonance radical. The relatively higher distribution of 9,11 and 10,12 isomers apparently results from the further stabilization of the c9,t11 or t10,c12 geometric isomers.
Advances in gas chromatography have enabled researchers to precisely determine the isomer composition of samples of CLA. In Christie et al., JAOCS 74 (11):1231 (1997), it was reported that the isomer distribution of a commercial sample of CLA was as follows: 8,10 (14%); 9,11 (30%); 10,12 (31%), and 11,13 (24%). In another study published by Christie et al. appearing in Lipids 33(2):217–21 (1998), the following CLA isomer composition of a commercial CLA preparation was reported: t11,t13 (0.74%); t10,t12 (1.23%); t9,t11 (1.18%); t8,t10 (0.46%); c11,t13 and t11,c13 (21.7%) c10,t12 and t10,c12 (29.0%); c9,t11 and t9,c11 (29.5%); c8,t10 and t8,c10 (12.3%); c11,c13 (0.96%); c10,c12 (0.88%); c9,c11 (0.88%); and c8,c10 (0.20%). As can be seen from these studies, even though the formation of certain isomers are favored, other isomers of CLA can contribute greatly to the composition of alkali isomerized CLA preparations.
In 1978, researchers at the University of Wisconsin discovered the identity of a substance contained in cooked beef that appeared to inhibit mutagenesis. This substance was found to be CLA. Fatty acids with conjugated unsaturation are not normally constituents of the cow's diet. However, c9,t11 octadecadienoic acid is formed in the rumen as a first intermediate in the biohydrogenation of linoleic acid by linoleic acid isomerase from the anaerobic bacterium Butyrivibrio fibrisolvens as reported by Kepler et al., J. Biol. Chem. 241:1350–54 (1966).
The biological activity of individual isomers of CLA has been the subject of some speculation. The literature generally suggests that the biologically active isomer is the c9,t11 isomer produced by Butyrivibrio fibrisolvens (for reviews adopting this position, see P. W. Parodi, J. Nutr. 127(6):1055–60 (1997), M. A. Belury, Nutrition Reviews 53(4):83–9 (1995)). Further data supporting this assumption appears in Ha et al., Cancer Res., 50:1097 (1991). There, the researchers conducted labeled uptake studies which indicate that the 9,11 isomer appears to be somewhat preferentially taken up and incorporated into the phospholipid fraction of animal tissues, and to a lesser extent the 10,12 isomer.
The biological activity associated with CLA is diverse and complex. At present, very little is known about the mechanisms of action of CLA, although several preclinical and clinical studies in progress are likely to shed new light on the physiological and biochemical modes of action. The anticarcinogenic properties of CLA have been well-documented. Administration of CLA inhibits rat mammary tumorigenesis, as demonstrated by Ha et al., Cancer Res., 52:2035-s (1992). Ha et al., Cancer Res., 50:1097 (1990), reported similar results in a mouse forestomach neoplasia model. CLA has also been identified as a strong cytotoxic agent against target human melanoma, colorectal and breast cancer cells in vitro. A recent major review article confirms the conclusions drawn from individual studies. (Ip, Am. J. Clin. Nutr. 66(6):1523s (1997)).
Although the mechanisms of CLA action are still obscure, there is evidence that some component(s) of the immune system may be involved, at least in vivo. U.S. Pat. No. 5,585,400 (Cook, et al.) discloses a method for attenuating allergic reactions in animals mediated by type I or TgE hypersensitivity by administering a diet containing CLA. CLA in concentrations of about 0.1 to 1.0 percent was also shown to be an effective adjuvant in preserving white blood cells. U.S. Pat. No. 5,674,901 (Cook, et al.) disclosed that oral or parenteral administration of CLA in either free acid or salt form resulted in elevation in CD-4 and CD-8 lymphocyte subpopulations associated with cell-mediated immunity. Adverse effects arising from pretreatment with exogenous tumor necrosis factor could be alleviated indirectly by elevation or maintenance of levels of CD-4 and CD-8 cells in animals to which CLA was administered. Finally, U.S. Pat. No. 5,430,066 describes the effect of CLA in preventing weight loss and anorexia by immune stimulation.
Apart from potential therapeutic and pharmacologic applications of CLA as set forth above, there has been much excitement regarding the use of CLA nutritively as a dietary supplement. CLA has been found to exert a profound generalized effect on body composition, in particular redirecting the partitioning of fat and lean tissue mass. U.S. Pat. No. 5,554,646 (Cook, et al.) discloses a method utilizing CLA as a dietary supplement in which pigs, mice, and humans were fed diets containing 0.5 percent CLA. In each species a significant drop in fat content was observed with a concomitant increase in protein mass. It is interesting that in these animals, increasing the fatty acid content of the diet by addition of CLA resulted in no increase in body weight, but was associated with a redistribution of fat and lean within the body. Another dietary phenomenon of interest is the effect of CLA supplementation on feed conversion. U.S. Pat. No. 5,428,072 (Cook, et al.) provided data showing that incorporation of CLA into animal feed (birds and mammals) increased the efficiency of feed conversion leading to greater weight gain in the CLA supplemented animals.
The potential beneficial effects of CLA supplementation for food animal growers is apparent. What is needed is a determination of what the actual biologically active isomers are and the appropriate ratios in which these isomers should be utilized.